Fix process pool fallback on Python 3.10

[MatthewScholefield]

Description

In Python 3.10 the exception generated by creating a process pool on a Python build that doesn't support this is now NotImplementedError.

Here is the stack trace I received on Python 3.10 on Termux / Android 11:

  File "/data/data/com.termux/files/home/my-project/.venv/lib/python3.10/site-packages/black/__init__.py", line 624, in reformat_many
    executor = ProcessPoolExecutor(max_workers=worker_count)
  File "/data/data/com.termux/files/usr/lib/python3.10/concurrent/futures/process.py", line 594, in __init__
    _check_system_limits()
  File "/data/data/com.termux/files/usr/lib/python3.10/concurrent/futures/process.py", line 542, in _check_system_limits
    raise NotImplementedError(_system_limited)
NotImplementedError: This Python build lacks multiprocessing.synchronize, usually due to named semaphores being unavailable on this platform.

Checklist

• Add a CHANGELOG entry if necessary?
• Add / update tests if necessary?
  • We could parameterize the test that injects OSError into ProcessPoolExecutor but at that point we're just repeating code.
• Add new / update outdated documentation?
  • n/a

[JelleZijlstra]

The 3.9 fuzz failure is expected. Thanks for the PR! I'll merge it as soon as the rest of CI passes and I'm on my computer.

[ichard26]

I'mma beat you to it Jelle :p

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Thank you so much for your contribution! This project is only possible by contributions like these 🖤. You're awesome, @MatthewScholefield. I do wonder how much black is really ran on systems without multiprocessing support like android 🤔